<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    //9. 将div#demo.demo转化成<div id="demo" class="demo"></div>
    let str = "div#demo.demo";
    str = str.replace(/^(\w+)#(\w+)\.(\w+)$/,function(match,$1,$2,$3){
        return "<"+$1+" id=\"" + $2+ "\" class=\""+ $3+ "\"><\/" + $1 +">"
    })
    console.log(str);
  </script>
</body>
</html>